Rapid determination of nitrate in vegetables by gas chromatography mass spectrometry

In this study we present a novel isotope dilution gas chromatography method for the determination of nitrate in vegetables. The analyte was extracted in water at 70^∘C and mixed with ¹⁵N isotopically enriched nitrate internal standard. The sample was centrifuged and the supernatant reacted with sulfamic acid for removal of nitrite, and with triethyloxonium tetrafluoroborate for converting nitrate into volatile EtONO₂. This simple aqueous chemistry allowed for separation of analyte from sample matrix in the form of a gaseous derivative which could be sampled in the headspace before GC-MS analysis. This key-feature of the method made possible the collection of clean chromatograms within an elution time of only 1.8 min. Detection of EtONO₂ could be performed using electron impact ionization with a standard GC-MS setup. The method was optimized and validated for the analysis of nitrate in fresh vegetables in the 10-10,000 μg/g range with a detection limit of 2 μg/g. Due to the use of primary isotope dilution quantitation, traceable results of high-precision were attained.